configure: look for a yacc program to be used by the testsuite

configure: look for a yacc program to be used by the testsuite

Instead of forcing the user to manually export 'YACC' in the
testsuite to use a non-bison yacc, we now look for a yacc program
at configure time, and use that as the default in the testsuite.

* configure.ac: Look for a yacc program, using AC_CHECK_PROGS.
* tests/defs.in: Updated to use the value of $YACC precomputed by
configure, unless the user overrides that in the environment.